Willem P. de Lange

Tsunami in Papua New Guinea was as intense as first thought

Last Julys tsunami in Papua New Guinea was as intense and catastrophic as news reports indicated, a scientific survey has found, and recommendations have been put forth to avert such a disaster in the future. The tsunami and the earthquake that generated it occurred July 17, 1998, and the International Tsunami Survey Team (ITST) began a weeklong investigation July 31. It was the ninth major tsunami and the most devastating the team has studied in the past 6 years. The team was able to precisely map the inundation and determine that media reports of extreme flows to fairly small sections of shoreline were accurate. Wave heights of 10 m were confirmed along a 25-km stretch of coastline with maximum heights of 15 m and overland flow velocities of 15–20 m/s. Both are extreme measurements, given the moderate size of the earthquake and its aftershocks.The team noted that the force of a tsunami current on an object is roughly 1000 times that of a wind of the same speed.

Volcano-meteorological tsunamis, thec. AD 200 Taupo eruption (New Zealand) and the possibility of a global tsunami

Meteorological tsunamis are long-period waves that result from meteorologically driven disturbances. They are also generated by phase coupling with atmospheric gravity waves arising through powerful volcanic activity. The AD 1883 Krakatau eruption generated volcano-meteorological tsunamis that were recorded globally. Because of its extreme violence and energy release (≥150±50 megatons explosive yield), and by analogy with the Krakatau event, it is highly possible that the ignimbrite-emplacement phase of the c. ad 200 Taupo eruption of North Island, New Zealand, generated a similar volcano-meteorological tsunami that may have reached coastal areas worldwide. Tsunami deposits of identical age to the Taupo eruption occur in central coastal New Zealand and probably relate to that event; definitive evidence elsewhere has not yet been found. In theory, volcano-meteorological tsunamis are likely to be produced during comparable eruptive events at other explosive volcanoes, and thus represent an additional volcanic hazard at coastal sites far from source. We suggest that evidence for such tsunamis, both for marine and lacustrine environments, may be preserved in geological records, and that further work searching for this evidence using a facies approach is timely.

Holocene sediment lithofacies and dispersal systems on a storm-dominated, back-arc shelf margin: The east Coromandel coast, New Zealand

Abstract Holocene sediment lithofacies and dispersal systems on the east Coromandel shelf, New Zealand, are mainly characterised by an accommodation-dominated regime in which autochthonous siliciclastic sediments were reworked through erosional shoreface retreat during the post-glacial marine transgression (12.0–6.5 ka). Stabilisation of sea level at its present position ca. 6.5 ka initiated onshore reworking of the autochthonous deposits into fine-grained regressive barrier and shoreface sands, while coarser sands remained offshore to form an erosional-lag inner-shelf deposit. Modern episodes of shoreface erosion rework fine and coarse autochthonous sands offshore and northwards into very large (η = 0.5–2.5 m; λ = 250–1500 m) submarine dunes. The submarine dunes are similar in form to sand ridges on the North American Atlantic shelf, but with crests striking perpendicular to both the shoreline and generating flows. Allochthonous siliciclastic lithofacies are also important aspects of east Coromandel shelf sedimentation, and are transported offshore from infilled estuary systems to form very fine-grained upper shoreface and muddy mid-shelf sands. The regional geology has a strong influence on shelf lithofacies and dispersal systems off the east Coromandel coast. Southern shelf regions are associated with deep back-arc basins which have formed major late Cenozoic depocentres for sediments sourced from the Taupo Volcanic Zone. Further north, a shallow volcanic platform extends out across the shelf and forms a barrier to any large-scale along-shelf dispersal of sediments. Consequently, lithofacies in northern shelf regions tend to be highly variable and reflect local catchment lithologies.

Coastal oceanography and sedimentology in New Zealand, 1967–91

Abstract This paper reviews research that has taken place on physical oceanography and sedimentology on New Zealands estuaries and the inner shelf since c. 1967. It includes estuarine sedimentation, tidal inlets, beach morphodynamics, nearshore and inner shelf sedimentation, tides and coastal currents, numerical modelling, short‐period waves, tsunamis, and storm surges. An extensive reference list covering both published and unpublished material is included. Formal teaching and research programmes dealing with coastal landforms and the processes that shape them were only introduced to New Zealand universities in 1964; the history of the New Zealand Journal of Marine and Freshwater Research parallels and chronicles the development of physical coastal science in New Zealand, most of which has been accomplished in last 25 years.

Debris dispersal modeling for the great Sumatra Tsunamis on Banda Aceh and surrounding waters

The Great Sumatra Tsunami on 26 December 2004 generated large amounts of debris and waste throughout the affected coastal region in the Indian Ocean. In Banda Aceh—Indonesia, the tsunami flows were observed carrying a thick muddy sludge that mixed with all kinds of debris from the destroyed buildings, bridges and culverts, vehicles, fallen trees, and other flotsam. This waste and debris was mostly deposited inland, but traveled both onshore and offshore. Numerical dispersal modeling is carried out to simulate the transport of debris and waste produced by the tsunamis during the event. The model solves the Lagrangian form of the transport/dispersion equations using novel particle tracking techniques. Model results show that understanding the pathway and distribution of the suspended materials and flotsam caused by tsunamis is important for a proper hazards mitigation plan and waste management action, and to minimize serious long-term adverse environmental and natural resources consequences.

Extreme Tsunami Run Up and Inundation Flows at Banda Aceh, Indonesia: Are There Any Solutions to this Type of Coastal Disaster?

Almost 3 years after the extreme 26 December 2004 tsunami event, which devastated the Banda Aceh region of Sumatra, certain coastal protection works have been planned and implemented. They comprise mainly of seawalls, breakwaters, and planting forest trees and mangroves as a wave-absorbing greenbelt. Numerical modeling of this catastrophic event was established to investigate the processes of tsunami propagation and inundation flows over the Banda Aceh coastal region, identify zones that had escaped devastation (as identified during the post event field survey) and assess possible types of coastal protection along the affected coasts. Results of the modeling showed that for protection against such a 2004-type event, huge structures ∼ 15 m high would be required, but are not economically feasible. However, mature mangroves stands may offer reasonable protection, and this option shows greater promise as an affordable solution.

Adjustments Toward Equilibrium of a Large Flood-Tidal Delta After a Major Dredging Program, Tauranga Harbour, New Zealand

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

Reliability of Geomorphic Indicators of Littoral Drift: Examples from the Bay of Plenty, New Zealand

ABSTRACT Healy, T. and de Lange, W., 2014. Reliability of geomorphic indicators of littoral drift: Examples from the Bay of Plenty, New Zealand. The net direction of littoral drift can be evaluated by multiple lines of evidence, including a range of geomorphic indicators and trends in sediment characteristics. In this article, the processes that influence the longshore transport of sediment are reviewed and assessed by the types of geomorphic indicators and sediment trends that can be expected. Various proxies for measured littoral drift are used to evaluate the littoral drift in the Bay of Plenty, New Zealand. Previous studies, utilising geomorphic indicators and, to a lesser extent, sediment characteristics and simple models of longshore sediment transport, developed a conceptual model of two main littoral drift systems starting at the western and eastern margins of the Bay of Plenty and converging in the vicinity of Ohiwa Harbour, an estuary enclosed by two opposing spits within the bay. Comparison of geomorphic indicators of littoral drift with the results of 30 years of monitoring and investigation demonstrate that geomorphic indicators can be misleading or misinterpreted. Overall, they do not appear to have been reliable for the Bay of Plenty, whereas sediment characteristics have performed better. The published rates of net littoral drift, and also the directions, appear to be too high and are inconsistent with observed patterns of erosion and accretion. Further, the exchange of sediment between the beach and the inner shelf has not been considered. Within the Bay of Plenty the net littoral drift magnitude and direction varies in response to changes in the wave climate associated with climatic variability, particularly the magnitude, frequency, and duration of the La Niña and El Niño extremes of the El Niño–Southern Oscillation. Decadal-scale fluctuations in the patterns of erosion and accretion along the Bay of Plenty coast have also been recognised. Finally, the influence of tectonic effects on geomorphic indicators of littoral drift needs further investigation.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Mörz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealands export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), quartz-feldspar sand and silt (UNIT5), middle pumiceous sand and silt (UNIT4), silt–sand–clay (UNIT3), upper pumiceous sand and silt (UNIT2), and Holocene marine (UNIT1) sediments forming the modern seafloor. Three paleovalleys filled with sediments from UNIT2 were identified extending west to east across the channel. The CPT profiles were correlated with the seismostratigraphic units and their corresponding soil behaviour type to characterise each units sediment stiffness. The UNIT6 is unlikely to be encountered in current and future capital dredging operations because it is the lowest observed unit. A special dredging methodology may be required for UNIT5 and UNIT4 as they have high CPT tip resistance and undrained shear strength. For UNIT3, UNIT2, and UNIT1, which have very low CPT tip resistance and undrained shear strength, trailing suction and cutter suction dredging is appropriate. The potential turbidity of the water column, however, could impose a significant threat to marine biota and has to be taken into account.

Spatial Patterns in Groundwater Seepage and Surf Zone Morphology: Muriwai Beach, New Zealand

ABSTRACT Emami, A.; Bryan, K.R., and de Lange, W.P., 2019. Spatial patterns in groundwater seepage and surf zone morphology: Muriwai Beach, New Zealand. Journal of Coastal Research, 35(1), 186–195. Coconut Creek (Florida), ISSN 0749-0208. Groundwater seeping from the beach face can induce erosion and so may play a role in controlling the development of beach morphology. A study at Muriwai Beach, New Zealand, was conducted to measure the relationship between surf zone morphology, groundwater, and intertidal beachface volume. Time exposure images from a video camera were used to study the surf zone morphology. Statistical analysis showed that the volume variation correlated with alongshore variation of the seepage line and surf zone morphology. Higher groundwater was always associated with lower volume. The surf zone morphology also correlated with changes in volume. This correlation was generally positive in winter (volume decreased with reduced pixel intensity) and negative in summer, suggesting a seasonal switch in the processes responsible for the correlation. To provide insight into the processes leading to this correlation, the sensitivity of the seepage line to hydraulic conductivity, beach morphology, tide, and wave setup was modeled using an exploratory model based on the Boussinesq formula. Results indicated that changes in setup played an important role in changing the seepage line location across the beach face. Spatial patterns in setup are likely to be driven by changes to wave breaking patterns caused by morphodynamic patterns. The results also showed that changes in conductivity and slope had a significant influence on seepage line variation. These findings indicated that alongshore variations in groundwater dynamics, which could in part be driven by offshore and onshore changes to water level, may play a stronger role in pattern development on beaches than previously expected.